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LTC3454EDD#PBF

P1-P3P4-P6P7-P9P10-P12
LTC3454
7
3454fa
OPERATION
Buck-Boost DC/DC Converter
The LTC3454 employs an LTC proprietary buck-boost
DC/DC converter to generate the output voltage required to
drive a high current LED. This architecture permits high-
effi ciency, low noise operation at input voltages above,
below or equal to the output voltage by properly phasing
four internal power switches. The error amp output voltage
on the V
C
pin determines the duty cycle of the switches.
Since the V
C
pin is a fi ltered signal, it provides rejection
of frequencies well below the factory trimmed switching
frequency of 1MHz. The low R
DS(ON)
, low gate charge
synchronous switches provide high frequency pulse width
modulation control at high effi ciency. Schottky diodes
across synchronous rectifi er switch B and synchronous
rectifi er switch D are not required, but if used do provide
a lower voltage drop during the break-before-make time
(typically 20ns), which improves peak effi ciency by typi-
cally 1% to 2% at higher loads.
Figure 1 shows a simplifi ed diagram of how the four internal
power switches are connected to the inductor, V
IN
, V
OUT
and GND. Figure 2 shows the regions of operation of the
buck-boost as a function of the control voltage V
C
. The
output switches are properly phased so transitions between
regions of operation are continuous, fi ltered and transpar-
ent to the user. When V
IN
approaches V
OUT
, the buck-boost
region is reached where the conduction time of the four
switch region is typically 150ns. Referring to Figures 1
and 2, the various regions of operation encountered as V
C
increases will now be described.
Buck Mode (V
IN
> V
OUT
)
In buck mode, switch D is always on and switch C is
always off. Referring to Figure 2, when the control
voltage V
C
is above voltage V1, switch A begins to turn on
Figure 2. Switch Control vs Control Voltage, V
C
10
SW1
6
SW2
PMOS A
NMOS B
9
V
IN
PMOS D
NMOS C
3454 F01
7
V
OUT
0%
DUTY
CYCLE
D
MAX
BUCK
D
MIN
BOOST
75%
D
MAX
BOOST
V4 (2.1V)
V3 (1.65V)
BOOST REGION
A ON, B OFF
PWM CD SWITCHES
D ON, C OFF
PWM AB SWITCHES
BUCK REGION
BUCK/BOOST REGION
V2 (1.55V)
V1 (0.9V)
CONTROL
VOLTAGE, V
C
3454 F02
FOUR SWITCH PWM
Figure 1. Simplifi ed Diagram of Internal Power Switches
each cycle. During the off time of switch A, synchronous
rectifi er switch B turns on for the remainder of the cycle.
Switches A and B will alternate conducting similar to a
typical synchronous buck regulator. As the control volt-
age increases, the duty cycle of switch A increases until
the maximum duty cycle of the converter in buck mode
reaches DC
BUCK
|Max given by:
DC
BUCK
|Max = 100% – DC
4SW
where DC
4SW
equals the duty cycle in % of the “four
switch” range.
DC
4SW
= (150ns • f) • 100%
where f is the operating frequency in Hz.
Beyond this point the “four switch” or buck-boost region
is reached.
Buck-Boost or 4-Switch Mode (V
IN
≈ V
OUT
)
Referring to Figure 2, when the control voltage V
C
is above
voltage V2, switch pair AD continue to operate for duty
cycle DC
BUCK
|max, and the switch pair AC begins to phase
in. As switch pair AC phases in, switch pair BD phases out
accordingly. When the V
C
voltage reaches the edge of the
buck-boost range at voltage V3, switch pair AC completely
phases out switch pair BD and the boost region begins at
duty cycle DC
4SW
. The input voltage V
IN
where the four
switch region begins is given by:
V
IN
= V
OUT
/[1 – (150ns • f)]
and the input voltage V
IN
where the four switch region
ends is given by
V
IN
= V
OUT
• (1 – DC
4SW
) = V
OUT
• [1 – (150ns • f)]
LTC3454
8
3454fa
APPLICATIONS INFORMATION
Boost Mode (V
IN
< V
OUT
)
In boost mode, switch A is always on and switch B is always
off. Referring to Figure 2, when the control voltage V
C
is
above voltage V3, switches C and D will alternate conduct-
ing similar to a typical synchronous boost regulator. The
maximum duty cycle of the converter is limited to 88%
typical and is reached when V
C
is above V4.
Forward Current Limit
If the current delivered from V
IN
through PMOS switch A
exceeds 3.4A (typical), switch A is shut off immediately.
Switches B and D are turned on for the remainder of the
cycle in order to safely discharge the forward inductor
current at the maximum rate possible.
Reverse Current Limit
If the current delivered from V
OUT
backwards through
PMOS switch D exceeds 275mA (typical), switch D is
shut off immediately. Switches A and C are turned on for
the remainder of the cycle in order to safely discharge the
reverse inductor current at the maximum rate possible.
Undervoltage Lockout
To prevent operation of the power switches at high R
DS(ON)
,
an undervoltage lockout is incorporated on the LTC3454.
When the input supply voltage drops below approximately
1.90V, the four power switches and all control circuitry are
turned off except for the undervoltage block, which draws
a few microamperes.
Overtemperature Protection
If the junction temperature of the LTC3454 exceeds 130°C
for any reason, all four switches are shut off immediately.
The overtemperature protection circuit has a typical hys-
teresis of 11°C.
Soft-Start
The LTC3454 includes an internally fi xed soft-start which
is active when powering up or coming out of shutdown.
The soft-start works by clamping the voltage on the V
C
node and gradually releasing it such that it requires 200μs
to linearly slew from 0.9V to 2.1V. This has the effect of
limiting the rate of duty cycle change as V
C
transitions
from the buck region through the buck-boost region into
the boost region. Once the soft-start times out, it can only
be reset by entering shutdown, or by an undervoltage or
overtemperature condition.
Autozero Error Amp
The error amplifi er is an autozeroing transconductance
amp with source and sink capability. The output of this
amplifi er drives a capacitor to GND at the V
C
pin. This
capacitor sets the dominant pole for the regulation loop.
(See the Applications Information section for selecting
the capacitor value). The feedback signal to the error
amp is developed across a resistor through which LED
current fl ows.
Safety Error Amp
The safety error amplifi er is a transconductance amplifi er
with sink only capability. In normal operation, it has no
effect on the loop regulation. However, if the LED pin open-
circuits, the output voltage will keep rising, and the safety
error amp will eventually take over control of the regulation
loop to prevent V
OUT
runaway. The V
OUT
threshold at which
this occurs is approximately 5.15V.
LED Current Programming and Enable Circuit
Two enable pins work in conjunction with dual external
resistors to program LED current to one of three nonzero
settings. The table below explains how the current can
be set.
EN1 EN2 I
LOAD
(A)
GND GND 0 (SHUTDOWN)
V
IN
GND 3850 • 0.8V/R
ISET1
GND V
IN
3850 • 0.8V/R
ISET2
V
IN
V
IN
3850 • (0.8V/R
ISET1
+ 0.8V/R
ISET2
)
With either enable pin pulled high, the buck-boost will
regulate the output voltage at the current programmed
by R
ISET1
and/or R
ISET2
.
With both enable pins pulled to GND, the LTC3454 is in
shutdown and draws zero current. The enable pins are
high impedance inputs and should not be fl oated.
LTC3454
9
3454fa
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3454 allows the
use of small surface mount inductors. The inductor cur-
rent ripple is typically set to 20% to 40% of the maximum
average inductor current. For a given ripple the inductance
term in boost mode is:
L
VVV
fI
IN MIN OUT IN MIN
OUT MAX
>
()
() ()
()
•– •%
•
2
100
••% •Ripple V
OUT
2
and in buck mode is:
L
VVV
fV Ripp
IN MAX OUT OUT
IN MAX
>
()
()
()
–••%
••%
100
ll e I
OUT
•
where f = operating frequency, Hz
%Ripple = allowable inductor current ripple, %
V
IN(MIN)
= minimum input voltage, V
V
IN(MAX)
= maximum input voltage, V
V
OUT
= output voltage, V
I
OUT(MAX)
= maximum output load current
For high effi ciency, choose an inductor with a high fre-
quency core material, such as ferrite, to reduce core loses.
The inductor should have low ESR (equivalent series
resistance) to reduce the I
2
R losses, and must be able
to handle the peak inductor current without saturating.
Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents >1A. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor. For white LED application, a 4.7μH/5μH
inductor value is recommended. See Table 1 for a list of
component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER WEB SITE
Coilcraft www.coilcraft.com
Cooper/Coiltronics www.cooperet.com
Murata www.murata.com
Sumida www.japanlink.com/sumida
Toko www.toko.com
Vishay-Dale www.vishay.com
Input Capacitor Selection
Since the V
IN
pin is the supply voltage for the IC it is recom-
mended to place at least a 2.2μF, low ESR bypass capacitor
to ground. See Table 2 for a list of component suppliers.
Table 2. Capacitor Vendor Information
SUPPLIER WEB SITE
AVX www.avxcorp.com
Sanyo www.sanyovideo.com
Taiyo Yuden www.t-yuden.com
TDK www.component.tdk.com
Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steady-
state ripple due to charge is given by:
%_
•– •
() ()
Ripple Boost
IVV
OUT MAX OUT IN MIN
=
()
100%%
••
%_
–•
()
CV f
Ripple Buck
VV
OUT OUT
IN MAX OUT
2
=
()
1100
8
2
%
••••
()
VfLC
IN MAX OUT
where C
OUT
= output fi lter capacitor, F
The output capacitance is usually many times larger in
order to handle the transient response of the converter. For
a rule of thumb, the ratio of operating frequency to unity-
gain bandwidth of the converter is the amount the output
capacitance will have to increase from the above calcula-
tions in order to maintain desired transient response.
The other component of ripple is due to ESR (equivalent
series resistance) of the output capacitor. Low ESR ca-
pacitors should be used to minimize output voltage ripple.
For surface mount applications, Taiyo Yuden, TDK, AVX
ceramic capacitors, AVX TPS series tantalum capacitors
or Sanyo POSCAP are recommended. For the white LED
application, a 10μF capacitor value is recommended. See
Table 2 for a list of component suppliers.
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but provide a lower drop during the
break-before-make time (typically 20ns) of the NMOS to
PMOS transition, improving effi ciency. Use a Schottky
P1-P3P4-P6P7-P9P10-P12

LTC3454EDD#PBF

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Buy LTC3454EDD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers Synch Buck-Boost High Power LED Driver in 3x3 DFN
Lifecycle:
New from this manufacturer.
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